Telephone and like system



y 1,1942. L. ,4. PADDLE 2,290,570

TELEPHONE AND LIKE SYSTEM Filed June 10, 1941 Patented July 21, 1942 UNITED STATES PATENT OFFICE Application June 10,1941, Serial No. 397,463 In Great Britain June '7, 1940 12 Claims.

This invention relates to telephone and like systems and more specifically to signalling apparatus therefor of the kind incorporating a circuit which functions to distinguish between signals intended for calling or ringing purposes and fortuitously occurring signals of the same frequency, for example voice frequency signals, and such a circuit hereinafter will be referred to as a voice immunity circuit.

The present invention has for its object to provide a signalling apparatus of the above character which is particularly simple but highly eifioient, and to this end a single space discharge valve device is utilised to effect amplification of modulated signalling currents and also functions in the voice immunity circuit.

Thus, according to one feature of the invention a signalling apparatus for a telephone system comprises a space discharge device having a cathode, at least one control electrode and two anodic electrodes, an input circuit coupled to a control electrode and adapted to feed thereto modulated signalling currents, an output circuit coupled to one of said anodic electrodes, means for extracting a signal component of modulation frequency from the amplified modulated signalling currents appearing in said output circuit, means serving to apply the said signal component to a control electrode, an output circuit coupled to the other anodic electrode, and means, responsive to the amplified signal component of modulation frequency which appears in said latter output circuit, for initiating a signalling operation.

In one embodiment of the invention a thermionic valve device of the pentode type is employed and the two anodic electrodes aforesaid are constituted by the screen grid and normal anode electrodes respectively. The control grid of this valve is adapted to be supplied with modulated signalling currents, for example by means of a circuit which is tuned to the frequency of the carrier, say 1,000 c. p. s. and there is coupled to the screen grid of the valve an output circuit which preferably is tuned to the carrier frequency and feeds a rectifying means. Thus there appears in the load circuit of the rectifying means a signal component which corresponds to the modulation frequency, say 17 c. p. s. and the whole or part of this signal component is applied to the control grid of the valve. To the anode of said valve there is coupled an output circuit in which there will appear amplified signal components of modulation frequency and preferably this output circuit is tuned to the modulation frequency. A relay device is arranged to be operated by current obtained from this latter output circuit, for example a rectifier means is adapted to provide a direct current component in response to signals of modulation frequency appearing in said circuit and this direct current is passed through the winding of a relay in order to operate contact means whereby a ringing or other signalling function is initiated.

In order that the invention can be fully appreciated it will now be described with reference to the accompanying drawing wherein:

Figure 1 shows schematically a circuit arrangement of one embodiment of the invention;

Figure 2 shows in similar manner another embodiment of the invention; and v Figure 3 shows, again in similar manner, the circuit arrangement of a further embodiment of the invention.

The arrangement shown in Figure 1 employs a thermionic valve device V of the pentode type and the control grid of this valve is connected with the secondary of a transformer Tl, the primary of which is adapted to be connected to a source of modulated signalling current. Such signalling current, for example, may be an alternating current carrier having a frequency of 1,000 c. p. s. which is modulated at 17 c. p. s., interruption of the carrier current being considered modulation.

The primary winding of a transformer T2 is connected to come between the screen grid of the valve V and the positive pole of a source of high tension adapted to be connected across the terminals HT. This primary winding is tuned to resonate at the carrier frequency.

The secondary winding of the transformer T2 is bridged by a rectifying network DI conveniently comprising rectifiers of the dry plate contact type and a load resistance RI, the latter being shunted by a condenser Cl. Said resistance Bi and the condenser Cl are given values such that there is obtained thereacross a signal component which is developed as a result of rectifying the amplified carrier current appearing in the output transformer T2, said component varying with the modulation of the carrier.

It will be observed that the resistance BI is serially connected with the secondary of the transformer TI and consequently the said signal component developed across the load resistance RI will be applied to the control grid of the valve V and any modulation of the carrier current will result in the application to the control grid of such a signal component varying at modulation frequency.

The anode of said valve V has connected thereto an output circuit comprising, as a load impedance for currents of modulation frequency, an inductance LI and a serially connected variable resistance RZ by Way of which the desired positive potential is applied to the anode of the valve. A condenser C2 serves to by-pass currents of carrier frequency.

The inductance LI and series resistance R2 are shunted by a rectifier D2 and a serially connected relay RL, the contacts of the latter being adapted to control a circuit serving to effect the desired ringing or other signal function. able resistance R2 enables the proportion of anode current flowing through the two circuits including the inductance LI and the relay RL respectively to be adjusted.

This particular arrangement serves to discriminate between true signalling currents and fortuitously occurring voice signals by reason of the fact that the true signal current (modulation component) has an initially steep wave front, whilst the fortuitously occurring signals usually present a more gradual increase to the maximum amplitude. Such discrimination and the general operation of the apparatus is as follows: Carrier wave signals applied to the control grid of the valve V by means of the transformer Tl will appear, in amplified form, in the transformer T2 and will be rectified by the rectifier network Dl to produce a direct current potential across the load resistance RI. This potential is applied to the: control grid of valve V with such polarity as to increase the electron current therethrough. Consequently when currents of carrier frequency are received the anode current of the valve V rises and causes an increase of current through the winding of the relay RL. By adjustment of the variable resistance R2 the proportion of anode current flowing through the inductance LI and the relay RL can be chosen so that a normal slow rise of anode current, such as would usually occur from the reception of voice signals of the same frequency as the carrier current, is just insufficient to cause the relay RL to operate. However in the event of the carrier being modulated with a signal component having a steep wave front, for example interruption of the carrier at 17 c. p. s., a sudden increase of anode current will take place in response to the rapid build-up of direct current potential across the load re.-

sistance RI and there will be a. momentary flick of A. C. potential, across the inductance Ll. This A, C. voltage will be rectified by the rectifi D2 n t incre sed flow of current through the winding of the relay will be suflicient to cause the operation of the latter and thereby close the associated contacts to initiate a desired signalling function. vibrate at the frequency of modulation or interr ption, assuming of course that this frequency is not too high.

Thus it will be appreciated that the arrangement serves to discriminate between true signalling currents and fortuitously occurring voice signals of the same frequency whilst moreover a single valve device is employed as an amplifier for the modulated carrier currents and at the same time functions in the voice immunity discriminating circuit.

The valve V may be biased so that grid current flows for large signal inputs, thus provid- The vari- 1 The relay will operate and ing a form of automatic compensation to render the apparatus suitable for a wider range of signals by reason of the fact that the anode current of the valve increases much less rapidly with the positive increase of grid potential.

According to a modification the rectifying network Dl is adapted to apply negative potentials to the control grid of the valve V in response to the reception of currents of signalling frequency and said grid normally is biased to obtain a large anode current so that upon receipt of an appropriate signal the anode current is reduced and releases the relay to bring about the desired signalling operation.

With minor modifications the voice immunity circuit incorporated in the arrangement of Figure 1 can be made to select the true signalling currents by reason of their depth of modulation. This modification will hereinafter be particularly described with reference to Figure 3.

In so far as the input circuits are concerned the arrangement shown in Figure 2 is in general similar to that of Figure 1 and readily will be appreciated by reference to the drawing wherein the corresponding elements have been similarly indexed. However in this case the output circuit of the valve V comprises a transformer T3 the primary of which is included in the anode circuit of the valve and is tuned to. resonate at the frequency of modulation of the carrier current whereby signal is to be effected. The secondary winding of this transformer T3v is connected with a rectifying network D3the load for which comprises the winding of a relay RL having contacts destined to control the local signalling circuit. Thus it will be observed that the valve V serves to selectively amplify the modulated carrier currents admitted by the transformer TI and also selectively amplify the modulation component, which latter, after such amplification, is applied to the rectifying network D3 to obtain a unidirectional current for operating the relay RI. It will be apparent therefore that in order to effect operation of the relay, signals of appropriate carrier frequency must be applied to the inputtransformer TI and moreover this carrier must be modulated at a frequency which corresponds with the tuning of the transformer T3 so as to assure the flow of an effective current through the relay winding. Consequently the arrangement offers quite a high degree of voice immunity.

In some cases it may be advantageous to avoid the possibility of the valve device running into grid current which may cause false operation of the system and for this purpose signal amplitude limiting means may be provided in association with the input circuit of the apparatus. Such an arrangement is shown in Figure 3 wherein the secondary of the input transformer TI is shunted by a limiter comprising a pair of oppositely connected diodes E and F biased by battery or other equivalent means G and H respectively. Alternatively the limiter may be connected across an additional secondary winding of the transformer Tl or the diodes maybe connected back to back across the secondary winding with a biasing voltage applied between the junction of the diodes and a centre tapping of said winding.

The arrangement of Figure 3 also shows how the modulation component of the signalling current can be parallel-fed to the control grid of the valve V as an alternative to the series-feed arrangement shown in Figures 1 and 2. Thus in this case the transformer T2, tuned to the carrier frequency, feeds the rectifying network DI, the load resistance RI of which is included as a shunt to the input circuit.

The output circuit for the modulation component comprises an anode circuit inductance L2 shunted by a condenser C3 constituting a tuned impedance adapted to resonate at substantially the frequency of modulation of the carrier current, for example in the case of a carrier current frequency of 1,000 c. 'p. s. modulated or interrupted at 17 c. p. s., said resonant circuit L2, C3 would be tuned to within plus and minus 20% of the frequency of 17 c. p. s. Said resonant circuit is shunted by a rectifier D6 arranged in series with a relay RL the contacts of which are adapted to control the local signal circuit. A variable resistance R3 is provided in the anode circuit in order that the proportion of the anode current flowing through the two parallel circuits containing the induct- I ance L2 and relay RL respectively can be varied.

Such an arrangement will operate to differentiate between true signalling current and fortuitously occurring voice signals of the same frequency by reason of a difference in the steepness of the initial part of the modulation component as has been more particularly described with reference to Figure 1. However by reason of the fact that the anode impedance comp-rises I the resonant circuit L2, C2 tuned to substantially the frequency of the modulation component, this circuit will be set into oscillation and the amplitude of oscillation will depend upon the peak amplitude of the signal pulses applied 7 thereto. Consequently the system can be made responsive to the depth of modulation of the signalling currents by setting the resistance R3 so that the current through the relay winding is not sufficient to operate said relay unless the As shown in Figure 3 the rectifier D4 is connected in such a manner that it conducts electron current in the anode circuit of the valve V, but it will be appreciated that it is also possible to obtain corresponding results if the rectifier is connected the other way round as is shown in Figure 1.

Furthermore and with reference to Figure 3 it will of course be understood that the signal pulses of modulation frequency applied to the control grid of the valve V as a result of the rectifying action of the network DI, may if desired be negative pulses instead of positive pulses.

As an alternative to applying to the control grid of the valve the component resulting from the rectifying action of the network D! in any of the arrangements described with reference to the accompanying drawings, only the A. C. component of modulation frequency appearing across the load impedance of this rectifying network may be transferred to the control grid.

In some cases it is found that the operation of the apparatus is improved by the inclusion of a high resistance in the input grid circuit of the valve. If then an input signal of very large amplitude is received, suflicient to overcome the standing negative bias on the grid and drives the grid into the region of grid current, there is the automatic limiting effect which helps to prevent false operation.

In some cases it may be advantageous to arrange for the circuit of the first output electrode to be adapted for operation a a signal frequency oscillator, thus effecting a further economy in the number of valves employed in the signalling system.

What I claim and desire to secure by Letters Patent is:

l. A signalling apparatus for a telephone system, comprising a space discharge device having a cathode cooperating with a control electrode and two anodic electrodes, means for applying modulated signalling currents to said control electrode, a rectifier circuit coupled to one of the anodic electrodes for extracting a modulation component from the amplified modulated signalling currents, means for applying said modulation component to said control electrode in order to modify the electron currentto the other anodic electrode, and means coupled to said other anodic electrode and responsive to such modification of the electron current for initiating a signalling operation.

2. A signalling apparatus for a telephone system, comprising a space discharge device having a cathode cooperating with at least one control electrode and two anodic electrodes, an input circuit coupled to a control electrode and adapted to feed thereto modulated signalling currents, means including an output circuit coupled to one of said anodic electrodes for developing a signal component of modulation frequency from the amplified modulated signalling currents appearing in said output circuit, means to apply the said signal component to a control electrode, an output circuit coupled to the other anodic electrode, and means responsive to the amplified signal component of modulation frequency which appears in said latter output circuit for initiating a signalling operation.

3. A signalling apparatus for a telephone system according to claim 2, the output circuit coupled to one of the anodic electrodes is tuned to the carrier frequency of the signalling currents so that the space discharge device serves to selecttively amplify the modulated signalling curren s.

4. A signalling apparatus for a telephone system according to claim 2, wherein said means for developing a signal component comprises means for amplifying the applied modulated signal currents, a rectifying means, and means to apply amplified modulated signalling currents to said rectifying means.

5. A signalling apparatus for a telephone system according to claim 2, wherein the second output circuit is tuned to the modulation frequency of the signalling currents so that the space discharge device serves to selectively amplify the modulation component of said signalling currents.

6. A signalling apparatus for a telephone system according to claim 2, wherein said space charge device has a common control electrode to which the modulated signalling currents and the modulation component developed therefrom are applied.

7. A signalling apparatus for a telephone system comprising a space discharge device having a cathode cooperating with control grid and anode electrodes, an input circuit for applying modulated signal currents to a control grid electrode, an output circuit connected to an anode electrode and including a transformer which is tuned to the carrier frequency of the modulated signal currents, rectifier means iconnected between said transformer and a control grid electrode to impress upon that grid electrode a pulsating voltage, and a second output circuit connected to an anode electrode, said second output circuit including signal controlling means responsive to the amplified pulsating voltage.

8. A signalling apparatus for a telephone system comprising a space discharge device having a cathode cooperating with control grid and anode electrodes, an input circuit for applying modulated signalling currents to a control grid electrode, an output circuit network coupled to an anode electrode and including a rectifier for developing a modulation component from the amplified modulated signalling currents, circuit means for applying the modulation component upon a control grid electrode, and an output circuit coupled to an anode electrode and including signal controlling means responsive to the amplified current output resulting from the modulation component input.

9. A signalling apparatus as recited in claim 8, wherein said signal controlling means is a relay; and said output circuit includes means for rectifying a sudden voltage impulse and impressing the rectified output upon said relay.

10. A signalling apparatus as recited in claim 8, wherein said output circuit includes an inductance, and said signal controlling means is shunted across said inductance and comprises rectifier means in series with a relay.

11. A signalling apparatus as recited in claim 8, wherein said output circuit includes an adjustable resistance for regulating the proportion of the output circuit current flowing to said signal controlling means.

12. In a telephone system of the type in which speech current and signalling currents deeply modulated at a voice frequency are transmitted over the same circuit, a signalling apparatus comprising a space discharge device having a cathode cooperating with control grid and anode electrodes, an input circuit for impressing speech currents and signalling currents between a control grid electrode and the cathode, an output circuit network coupled to an anode electrode and including a circuit resonant at the carrier frequency of the modulated signalling currents, reflex circuit connections including rectifier means between said resonant circuit and a control grid electrode, an output circuit including a signal controlling device connected between the cathode and an anode element, and means adjustable to render said signal controlling device responsive only to modulation frequency currents of steep wave front.

LESLIE HAROLD PADDLE. 

